Replace O(n) `Query::iter().find()` card scans with O(1) `CardEntityIndex`
lookups in the mouse and touch drag pipelines (`follow_drag`, `end_drag`,
`touch_follow_drag`, `touch_end_drag`) and `update_drag_shadow` — 7 sites
across 5 systems. Each ran per dragged card per frame during a drag.
`InputPlugin` now defensively `init_resource::<CardEntityIndex>()` (idempotent;
`CardPlugin` still owns and rebuilds it) so the plugin is self-sufficient in
tests. The lone remaining card-keyed `.find` is a `#[cfg(test)]` world-query
helper, which is the correct pattern there.
Completes the CardEntityIndex migration started in ef1efdc.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
funman300
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